The only thing I could muster as an in-between was "I probably use less fuel this way." She didn't say anything and it ended.
She didn't say anything 'cuz she got shit on and/or doesn't understand fuel efficiency. It's hard to argue with the truth.
Given the situation, I think this was the most reasonable response.
I wouldn't say you got "punked in public" dude.
Plenty of people around here leave their car running when they go into the local mini mart in the morning to get their coffee. Completely normal thing to do.